Field of the Invention
This invention relates to a dual polarized microstrip patch antenna and, more specifically to such an antenna for use in wireless, cellular and personal communication system (PCS) base stations.
Brief Description of the Prior Art
In mobile communications, the signal from a mobile station arrives at the base station from many directions. There are many reasons for this multidirectional reception, examples being reflection of the signal from buildings, terrain and the like of at least a portion of the signal disposed in the path of the signal from the mobile station to the base station. Signals arriving at the base station from different directions add in different amplitudes and phases, causing signal fading. The fading characteristics depend upon the speed of the mobile station and the character of the surrounding reflective obstacles. Signals can fade by as much as 40 dB, thereby seriously degrading communication performance.
A common approach to reduce the effects of fading involves reception of signals through several antennas configured appropriately so that received signals tend to be uncorrelated. The distinct antenna signal paths are termed diversity branches. A traditional approach to create diversity branches is to spatially separate the antennas by several carrier signal wavelengths (usually 5 to 10 wavelengths). The signals received by such antennas tend to be decorrelated (i.e., when the signal in one branch goes through a null, the signal in the other branch goes through a peak). The signals in these diversity branches are combined using either switched, equal gain or maximum ratio combining techniques.
The diversity technique used here involves the use of orthogonal polarizations from a common antenna. When the signal (linearly polarized) from a mobile station reaches a base station through a random scattering medium, the signal loses its dominant polarization and the incident power is scattered in all possible polarization states as a function of time. The sampled vertical and horizontal polarization components of the incident field thus provide independent time varying signal paths. The degree of decorrelation depends upon the propagation medium. Thus, in a densely urban area, the orthogonal polarizations are more uncorrelated than in a rural area.
A technique for improving communication has been to receive a signal from an antenna having dual orthogonal polarizations. The traditional method for achieving dual polarization (e.g., vertical and horizontal polarization) from a compact antenna structure has been to provide dual feeds, generally orthogonal to a common radiating element as shown in FIGS. 1a and 1b wherein the feeds are vertical and horizontal in FIG. 1a and at 45.degree. and 135.degree. relative to the horizontal in FIG. 1b. The radiating element can take the form of a printed circuit with generally rectangular patches disposed on a surface of an electrical insulator. The patches must be spaced apart from patch center to patch center by less than one wavelength of the carrier frequency and preferably about 0.75 wavelength which is the spacing where peak amplitude reception is generally obtained. The patches are dimensioned to accommodate a predetermined carrier frequency as is well known. The patch is generally fed by either a direct coupled feed on the surface containing the patch to provide signals Lo the patch, a probe feed wherein separate probes extend through the printed circuit board to the patch or by providing a microstrip line on the opposing surface of the printed circuit board with slots to provide slot coupled feeds to the patch, in each case to provide for orthogonal polarization modes.
A common drawback to these prior art approaches using a patch antenna is that the coupling between adjacent feeds limits the level of cross-polarization that can be achieved. Also, it is difficult to achieve equal E and H plane pattern shapes from a common antenna element. Any modifications performed to alter the pattern shape of one polarization tends to detune the resonant frequency of the other polarization.